Fix search for create_table_migration template override

A developer should be able to customize the template used by the model or migration generator, by adding a file to their source tree in a predictable location.

This functionality was likely broken by:
https://github.com/rails/rails/commit/20e041579f5fe9df51bdc40048cc1ef78915e116

After that change, you could now generate a migration:

`rails g migration CreateFoo`

and it would look for a custom template at `myproject/lib/templates/active_record/migration/create_table_migration.rb`

However, if you generated the migration by generating a model:

`rails g model Bar`

it would look for a template at the unpredictable and awkward path `myproject/lib/templates/migration/templates/create_table_migration.rb`


This Pull Request adds the migration generator's template paths to the model generator's search paths. When you generate a model, it will now look for templates in order:

```
<myproject>/lib/templates/active_record/model/create_table_migration.rb
<activerecord gem>/lib/rails/generators/active_record/model/templates/create_table_migration.rb
<myproject>/lib/templates/active_record/migration/create_table_migration.rb
<activerecord gem>/lib/rails/generators/active_record/migration/templates/create_table_migration.rb
```

This has the side-effect of allowing you to have different templates for creating a table, depending on whether you generated a model or the migration directly.